Use correct element for Form Not Secure warning

Web forms may not contain a username element, therefore it is not safe
to assume that such an element exists when calling ShowNotSecureWarning.
Instead, use the actual element found by the autofill logic.

BUG=678769

Review-Url: https://codereview.chromium.org/2612893003
Cr-Commit-Position: refs/heads/master@{#441988}

components/autofill/content/renderer/form_autofill_util.cc

@@ -1619,6 +1619,8 @@ bool UnownedPasswordFormElementsAndFieldSetsToFormData(
 bool FindFormAndFieldForFormControlElement(const WebFormControlElement& element,
                                            FormData* form,
                                            FormFieldData* field) {
+  DCHECK(!element.isNull());
+
   if (!IsAutofillableElement(element))
     return false;
 

components/autofill/content/renderer/password_autofill_agent.cc

@@ -1266,7 +1266,7 @@ void PasswordAutofillAgent::FillPasswordForm(
                        base::Unretained(&gatekeeper_)),
             logger.get())) {
       if (form_data.show_form_not_secure_warning_on_autofill)
-        autofill_agent_->ShowNotSecureWarning(username_element);
+        autofill_agent_->ShowNotSecureWarning(element);
     }
   }
 }